This application is based on French Patent Application No. 00 13 214 filed Oct. 16, 2000, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the Invention
The present invention relates to an aqueous alkaline electrolyte Ni-MH sealed secondary storage cell of high capacity and low internal pressure, including a bundle of electrodes placed in a container and made up of a plurality of negative and positive electrodes, the space separating the negative electrode from the positive electrode containing a separator, in which cell a recombination device is placed between two adjacent negative electrodes.
2. Description of the Prior Art
Aqueous alkaline electrolyte storage cells include Ni-MH cells, i.e. nickel and hydridable metal alloy or metal cells. Industrial sealed storage cells have a high capacity, from 5 Ah to 400 Ah.
In storage cells, electrochemical reduction (recombination) of the oxygen produced by the positive electrode on overcharging occurs at the surface of the negative electrodes. In industrial storage cells, the low permitted pressures, typically less than 2 bar, and the higher inter-electrode distance, as well as a greater quantity of electrolyte, mean that direct recombination only at the negative electrodes facing the positive electrodes is insufficient. Recombination devices are therefore provided at the negative electrodes to accelerate recombination.
The documents FR-1 243 784, U.S. Pat. No. 3,023,258 and U.S. Pat. No. 5,447,806 describe recombination devices in the form of spacers between two negative half-electrodes, that spacer allowing oxygen which is evacuated into the space at the top of the cell (and which causes the overpressure) to recombine on the surfaces defined by the spacer. In the case of the document U.S. Pat. No. 5,447,806 in particular, which is specifically devoted to Ni-MH storage cells, the spacer is formed by a porous metal or plastics material structure. According to the documents FR-1 243 784 and U.S. Pat. No. 3,023,258, which are older (and therefore relate to alkaline storage cells but not specifically to Ni-MH cells), the spacer is made of nickel, for example. In the document U.S. Pat. No. 5,447,806 the separators used are impermeable to gases.
Storage cells of the type constituting the subject matter of the document U.S. Pat. No. 5,447,806 suffer from numerous defects.
The (gas-impermeable) separators used have a much higher resistivity than highly porous separators (such as non-woven materials, for example, for which the resistivity for the same thickness is of the order of one fifth).
For industrial Ni-MH storage cells developed for electric vehicle applications, for example, the characteristics of these prior art storage cells cannot achieve pressures lower than the relief valve opening pressure ( less than 3 bar) and therefore necessitate maintenance (regular topping up with water); the reason for this is that the conditions described in the document U.S. Pat. No. 5,447,806 cannot achieve low pressures for small dead volumes (to increase the energy per unit volume) and high quantities of electrolyte (to increase service life).
An object of the invention is to provide a maintenance-free Ni-MH secondary storage cell with a relief valve that opens at a very low pressure (in particular less than 3 bar), a small dead volume (to optimize the energy per unit volume), and a high quantity of electrolyte (for a high service life).
This object is achieved by using a hydrophobic and porous recombination device of minimum thickness.
The invention therefore provides an aqueous alkaline electrolyte Ni-MH sealed secondary storage cell including a bundle of electrodes placed in a container and made up of a plurality of negative and positive electrodes, the space separating the negative electrode from the positive electrode containing a separator, in which cell the separator is permeable to gases and a recombination device is placed between two adjacent negative electrodes and has a wetting angle of at least 45xc2x0, an average pore section of at least 103 xcexcm2, and a thickness from 0.2 mm to 5 mm.
In one embodiment the wetting angle of the recombination device is at least 60xc2x0.
In one embodiment the thickness of the recombination device is from 0.3 mm to 2 mm.
In one embodiment the recombination device is made of polyethylene or polypropylene.
In one embodiment the recombination device is non-conductive.
In one embodiment the recombination device is surface-treated with PTFE.
In one embodiment the cell has an operating pressure less than 2 bar.
In one embodiment the cell has a dead volume less than 2 cc/Ah.
In one embodiment the cell has a quantity of electrolyte greater than 1.8 cc/Ah.
In one embodiment the cell has a capacity per unit volume greater than 100 Ah/l.